1. Field of the Invention
The present invention relates to a fundus camera that includes an automatic focusing unit.
2. Description of the Related Art
There is conventionally known a fundus camera that includes an illumination optical system configured to illuminate a fundus of subject's eye, a photographic optical system configured to photograph the fundus, an observation optical system configured to observe the fundus, and a focusing optical system configured to focus on the fundus. Photographing the fundus necessitates complex work and experience such as performing alignment with a pupil of subject's eye, working distance adjustment, focusing, and line-of-sight guiding, and paying attention to rising or blinking of an eyelid.
Thus, there is used a fundus camera that facilitates fundus photographing by automating focusing. Automatic focusing systems are largely classified into two types: a detection method of projecting a focusing index to the fundus and capturing and processing its index image, and a method of detecting a blur of a captured fundus image without using any focusing index.
Japanese Patent No. 4138101 discusses a fundus camera in which, in a focusing index projection system, eye refractive power of subject's eye is input before fundus photographing, a focusing lens is moved to a suitable position, and then focusing is started. This arrangement is for the purpose of solving a problem of a focusing time delay caused by a longer period of moving time of the focusing lens when refractive power is large.
In the case of the fundus camera that performs focusing control by the fundus image blur detection method, if the focusing lens is roughly moved to an eye refractive power position of subject's eye beforehand, the focusing lens is moved to the vicinity of a vertex of a focusing evaluation value curve.
However, a shape of the focusing evaluation value curve obtained from the fundus image is not sharp. Thus, even when after the focusing lens is moved to the vicinity of the vertex, the focusing evaluation value is acquired to calculate an in-focus position, it is difficult to determine in which side the peak of the mountain exists. At least, a mountain of the focusing evaluation value curve is scanned nearly from a mountainside to understand an overall shape of the mountain, and then the vertex is found. Otherwise, no accurate focusing result can be obtained.
Thus, in the conventional fundus camera, the focusing lens is first moved to a −end side of a moving range, and then scanning is started to acquire a focusing evaluation value in a +end direction. After detection of a mountain peak of the focusing evaluation value, the scanning is stopped when the focusing evaluation value becomes equal to or less than a given setting value. Then, the focusing lens is moved back to a peak position of the focusing evaluation value to perform photographing.
In FIG. 10, a horizontal axis represents a moving range of the focusing lens, and a +end indicates a limit position of a plus diopter, and a −end indicates a limit position of a minus diopter. A vertical axis (plus side) represents a focusing evaluation value obtained by adding a value of a high-frequency component obtained in a predetermined area of the fundus image. Contrast is higher near the in-focus position, and then the focusing evaluation value is larger.
A focusing evaluation value curve of FIG. 10 is an example of a focusing evaluation value when the focusing lens is scanned within the entire moving range. Actually, only a curve of a scanned range is obtained. A lower side of the vertical axis of FIG. 10 indicates a sequence of focusing lens movement and focusing evaluation value scanning on a time axis.
A solid line Mx indicates a case where only movement of the focusing lens is performed, and a broken line Sx indicates a case where scanning for focusing evaluation value acquisition is performed, x indicating order. The graph also illustrates a start position 301 of focusing lens control, and a focusing end position 301. Normally, photographing is performed in the focusing end position 302.
In this example, the focusing lens moves from the position 301 to the −end, starts scanning S1 to acquire a focusing value from M1 in a +direction, detects a peak of a focusing evaluation value curve on the way, stops when a focusing evaluation value becomes equal to or less than a threshold value L0, and moves (M2) to perform photographing at a peak position. A moving speed of the focusing lens is, in an operation Sx that is accompanied by focusing evaluation value acquisition, greatly lower than that of movement Mx where only movement is performed.
However, such a fundus camera is based on the premise that eye refractive power of subject's eye is known beforehand, and a unit for inputting the eye refractive power is necessary. Consequently, the number of operation units increases, causing a cost increase. In the case of the fundus camera that performs focusing control by the blur detection focusing method, scanning for a focusing evaluation value is performed after moving to the −end position (or +end) of the focusing range, and hence it takes time to focus.